We're not looking to create any new Pixelmator localizations and we usually work with one set of localizers (it makes the whole process much simpler), but I really do appreciate the offer and sentiment!

The reasons are related mostly to quality control, really — text elements in the appx often change and ensuring everything is always translated on time and with an acceptable level of quality would be tricky with a crowdsourced/volunteer effort, so we generally stick with professional services, though it doesn't really make economic sense for us to have a very wide range of localizations. Though I know how the feeling, I'd love it if Apple's operating systems were available in Lithuanian, for example, so I could get my gran an iPad. Alas...

Ah, if we were to use an external plug-in method for which we weren't responsible, that would be a little different, sure. I was under the impression that this would be something that we'd include ourselves. At this time though, when it comes to external plug-ins, Pixelmator supports Quartz Composer compositions, which appear as effects in the effect browser and basically let you create more visual effects. It would not be possible to add a translation in this way. As for making it possible to support extensions and plug-ins, Pixelmator wasn't created with this idea in mind and adding support for it would require some serious engine changes. I personally quite like the idea but it's not quite as simple as flipping a switch and would actually require extensive development.

I read that your code base was around twelve years old and that the "Pro" version was a re-write which would allow you to build for the future with the knowledge you'd learnt up to this point.

Plug-ins can be monetised and this could be a great future to position yourself in. Apple gave developers $120bn in just over a decade, of which they took 30% for themselves. If you cast your mind back to IE, when it had 98% browser market share, the one thing that broke evil Microsoft's monopoly was FireFox's ability to allow new features via plug-ins and the reason people flocked to it.

Apple's Quartz filters have been around for ages, they're basically royalty free now. I remember being at a trade show and one developer for PS had a filter that would highlight between strands of hair and it cost ~$200 (which they were confident professionals would pay), I was astonished, but if I look at Apple's App store I see plenty going for 70p, in fact I will for myself soon what people will pay.

I think that the "Pro" codebase should consider plug-ins (along with the security needed for this) and the benefits that come from allowing the community to expand your software.